Most flowering plants produce perfect flowers containing both the male organs (stamens) and female organs (pistils). In maize, which has physically separated male and female inflorescences, floral meristems become unisexual through sex determination (Dellaporta & Calderon-Urrea, 1994, Science 266:1501; Irish, 1996, Bioessays 18:363). The basic unit of the maize inflorescence, called a spikelet, contains one upper and one lower flower (known as florets in grasses). Each floret initiates a series of floral organs including three stamen primordia and a central pistil primordium (Bonnet, 1940, J. Agric. Res. 60:25; Kiesselbach, “The Structure and Reproduction of Corn,” Univ. of Nebraska Press, Lincoln, Nebr., 1949). These initially bisexual florets become exclusively staminate in the tassel (by abortion of pistil primordia) and exclusively pistillate in the ear (by arrest of developing stamens) (Bonnet, 1940, J. Agric. Res. 60:25; Cheng et al., 1983, Am. Bot. 70:450). Each ear spikelet produces a solitary functional pistil in the upper floret due to abortion of the pistil in the lower floret (Bonnet, 1940, J. Agric. Res. 60:25; Kiesselbach, “The Structure and Reproduction of Corn,” Univ. of Nebraska Press, Lincoln, Nebr., 1949; Cheng et al., 1983, Am. Bot. 70:450).
Mutations altering the sexual fate of florets in maize indicate that sex determination is under genetic control. The non-homeotic tasselseed (ts) mutations ts1 and ts2 result in the conversion of the tassel inflorescence from staminate to pistillate (Emerson, 1920, J. Hered. 11:65; Nickerson & Dale, 1955, Ann. Mo. Bot. Gard. 42:195). Both ts1 and ts2 are required to eliminate pistil primordia through cell death (Calderon-Urrea & Dellaporta, 1999, Development 126:435; Kim et al., 2007, Genetics 177:2547). The ts2 gene encodes a short-chain dehydrogenase/reductase (DeLong et al., 1993, Cell 74:757) with broad activity, which has complicated the discovery of its natural substrate (Wu et al., 2007, FEBS J. 274:1172). It is unknown how is genes mediate pistil cell death, although it has been suggested that the dehydrogenase/reductase activity of ts2 may produce a pro-apoptotic signal or metabolize a substrate required for cell viability (Calderon-Urrea & Dellaporta, 1999, Development 126:435; Wu et al., 2007, FEBS J. 274:1172). Even less is known about the ts1 gene. TS2 transcripts are low or undetectable in ts1 mutant tassels, which suggests that ts1 may act upstream of ts2 by regulating ts2 RNA levels and possibly other sex determination genes (Calderon-Urrea & Dellaporta, 1999, Development 126:435).
Chemicals have been used to modulate and modify sexual differentiation in plants. The plant hormone ethylene has been observed to promote feminization in cucumber (Yamasaki et al., 2005, Vitam. Bonn. 72:79). Recent genetic and biochemical evidence has confirmed the role of ethylene in sex determination of melon, a related species (Boualem et al., 2008, Science 321:836). Conversely, gibberellin has masculinizing effects in cucumber but promotes feminization in maize (Bensen et al., 1995, Plant Cell 7:75), and auxin also has opposing effects in cucumber and Mercurialis annua (Yamasaki et al., 2005, Vitam. Horm. 72:79). However, little is known at this point about the role that chemicals play in sexual development and maturation of plants.
There is thus a great interest in identifying chemical compounds that modulate sexual differentiation in plants. Such compounds would be useful in suppressing or enhancing specific sexual phenotypes in plants, allowing the control of vast groups of plants without the need for time-consuming mechanical manipulation of the plants. The present invention fulfills these needs.